A fifth wheel is a device that is mounted on the rear bumper of a vehicle and is used to make very accurate measurements of the speed and distance travelled of the vehicle. It comprises a very accurate and sensitive speedometer and odometer driven by a large bicycle wheel; since it is not used for long distance measurements it does not have to be very rugged or durable. Therefore durability is sacrificed for accuracy, and the cost is correspondingly higher than the cost of the speedometer that is in the car; some fifth wheels cost about $2,000 each.
As long as the vehicle undergoes just straight line testing there is no problem with the fifth wheel. A problem arises, however, when the vehicle is backed up for more than a short distance or spins out in the course of a test. Under these conditions the fifth wheel is moved in a horizontal arc relative to the rear of the vehicle and if the arc is large enough will slam into the rear of the vehicle; this results in severe damage to the fifth wheel, and can also inflict slight damage to the sheet metal of the vehicle.
Solving the above problem by merely limiting the horizontal travel of the fifth wheel is not practical because if this is done, the wheel will be locked in position at an angle to the vehicle and will be skidded along behind it. This will at the minimum wear the tire in one spot, causing a flat spot on it, and may result in the wheel itself being bent out of shape. Furthermore, it will not allow the fifth wheel to exactly follow the vehicle through tight turns and will thus reduce the accuracy of the test. What is needed is a simple, reliable way of preventing damage to the fifth wheel under the above circumstances.